1. Field of the Invention
The present invention relates generally to test equipment for measuring electromagnetic emission in mid- and far-infrared spectral ranges and relates more specifically to a blackbody cavity for calibration of infrared thermometers.
2. Description of Prior Art
Temperature of an object can be measured by means of thermal radiation when naturally emanated electromagnetic (thermal) radiation in the mid- and far-infrared (IR) spectral ranges is detected by an appropriate sensor. The IR sensor output signal is indicative of the surface temperature of an object. A sensor is an integral part of a measurement and/or data acquisition system. As a rule, a sensor is combined with some kind of an optical system which may include refractive and reflective components, such as lenses, mirrors, wave guides, windows, among others. A combination of optical components, an infrared sensor, and signal conditioning electronic circuits produces a useful signal which corresponds to the surface temperature of an object of measurement. Before the signal can be converted into any meaningful temperature number, its relationship with the IR radiation magnitude must be established with high accuracy. A standard way to find such a relationship is to calibrate an IR thermometer by exposing it to a well defined IR radiation source. To provide such a radiation source, a calibrating device known as a xe2x80x9cblackbodyxe2x80x9d is routinely used by IR thermometer manufacturers.
In general, a blackbody is characterized by two fundamentally essential traits: temperature and emissivity. Temperature must be known with a required uncertainty, while emissivity must be as close to the value of unity as possible. Unity is a theoretical limit which describes an ideal blackbody. When a blackbody is designed, a prime goal is to approach the unity emissivity as close as practically possible. For example, a value near 0.999 would be considered well acceptable for calibrating most modern infrared thermometers.
As a rule, metals are poor emitters of IR radiation, that is, their emissivities are well below unity. A majority of non-metallic surfaces have relatively high emissivities. For example, organic paint, regardless of its color visible to a human eye, in mid- and far-infrared spectral ranges has emissivity of about 0.95. This is not nearly enough to consider a painted surface as a blackbody. In physics, there is a well established method to artificially increase the emissivity of a material. It is based on the so called xe2x80x9ccavity effect.xe2x80x9d The effect is described, for example, in J. Fraden, xe2x80x9cHandbook of Modem Sensors,xe2x80x9d American Institute of Physics, pp. 112-114 (1997), and is used by all producers of blackbodies. When IR radiation is emanated not from a flat surface but rather escapes from a small opening (aperture) in a cavity of a uniform wall temperature, the effective emissivity at the aperture becomes much higher then that of the internal cavity walls. This is due to the multiple internal reflections of IR radiation inside the cavity. Frequently, a blackbody cavity is made of metal having high thermal conductivity with a non-metallic coating on its internal cavity. A typical cavity blackbody has emissivity in the range from 0.980 to 0.990 which often is still not close enough to an acceptable level of 0.999.
A challenge to a blackbody designer is to provide a cavity having uniform wall temperatures with as many internal reflections as possible. This is often accomplished by making a cavity in a shape of a cone and immersing it into fluid, like water, of known temperature. (See, for example, U.S. Pat. No. 5,183,337, issued to Pompei.) To increase the number of internal reflections, a ratio of a cavity wall surface to the aperture shall be maximized. In many instances, this is not easy to accomplish, because any increase in cavity dimensions makes it much more difficult to assure the uniform wall temperatures, which for high precision blackbodies should not vary by more than 0.02xc2x0 C. over the entire cavity surface. The most troublesome portion in the cavity is it""s front end where the IR thermometer is inserted in the aperture. Dimensions of the front end should allow placement of the thermometer probe into the cavity opening, which may affect cavity wall temperatures near the opening and subsequently degrade the effective emissivity.
Thus, a primary purpose of the present invention to provide a blackbody having emissivity approaching unity. A further object is to provide a blackbody cavity which is less influenced by variations in environmental temperature. It is another object of the invention to provide a blackbody cavity having smaller dimensions. And it is a further object to provide a blackbody cavity which is easy to manufacture and maintain. Naturally, not every embodiment of the present invention will achieve all of these objects and advantages.
A blackbody cavity is disclosed having two types of wall surfaces wherein a first type has high emissivity and a second type has low emissivity. The low emissivity wall surface has an aperture from where the infrared radiation escapes the cavity, and is preferably shaped to minimize the escape through the aperture of radiation emanated directly from the low emissivity wall itself. The combination of high and low emissivity wall surfaces allows the blackbody to reduce the influence of the environmental temperature while maintaining emissivity approaching unity.